core: Keep track of per pointer/keyboard grabs

There is now a MetaGrabInfo struct, holding all information about
an ongoing grab, there were unfortunately no means of making this
commit smaller, as things are too intertwined to separate it,

On places where it was most obvious, the correct device has been
used (in reaction to events, for example), in other parts, the
private API has been extended to include the MetaDevice (or device
ID for core.h functions), in other places, the virtual core
pointer/keyboard are used out of cluelessness.

src/core/frame.c

@@ -382,8 +382,7 @@ meta_frame_sync_to_window (MetaFrame *frame,
       /* If we're interactively resizing the frame, repaint
        * it immediately so we don't start to lag.
        */
-      if (frame->window->display->grab_window ==
-          frame->window)
+      if (frame->window->cur_grab != NULL)
         meta_ui_repaint_frame (frame->window->screen->ui,
                                frame->xwindow);
     }